Ion source devices, such as hollow cathode devices, are commonly used when it is desired to ionize a source material. The source material may be a gas, such as argon or xenon, or the vapor which results from heating a substance such as cesium or mercury. A hollow cathode device is formed typically of a hollow metallic tube which is in electrical contact with the negative terminal of a power supply. An electron current, in the form of an arc, flows between the hollow cathode and an electrode, or anode, which is connected to a positive terminal of the power supply. The gas or vapor source material passes through the hollow cathode and exits an end where it is ionized within the arc creating, in the case of mercury vapor, positively charged mercury ions and free electrons. The electron current flowing between the hollow cathode and the anode is known as the emission current of the cathode. The magnitude of the emission current is a function of several factors, two of which are the magnitude of the voltage of the power supply, and, the amount of source material available to be ionized.
One valuable application for hollow cathode devices is as a component of an ion propulsion motor, or ion thruster. One application of ion thrusters is for attitude control of satellites and other spacecraft. The thrust available from an ion thruster is typically significantly less than that available from traditional chemical propulsion motors, therefore limiting their use to the vacuum conditions found in space. However, their specific impulse, which is typically defined as the thrust per unit weight flow of exhaust gasses, is significantly higher than that of a traditional chemical propulsion motor, typically by as much as an order of magnitude. In other words, the fuel required for a given level of thrust weighs ten times less than the fuel required in a chemical impulse motor. Such significantly smaller fuel requirements make ion propulsion motors well suited for applications where low levels of thrust are acceptable over long periods of time. In an ion propulsion motor the gas or vapor fed to the hollow cathode is typically referred to as the propellant.
Below a certain level of thrust, a single hollow cathode device may be sufficient to provide the emission current required by the motor. However, as ion thrusters are scaled upwards to achieve larger amounts of thrust, a problem arises in that the emission current capacity of a single hollow cathode device may be exceeded. A proposed solution of this problem has been the inclusion of one or more additional hollow cathode devices within the ion propulsion motor, each device thereby contributing to the total emission current and, hence, thrust of the motor.
The use of more than one hollow cathode device, however, can create additional problems in the design and operation of a higher thrust ion propulsion motor.
One problem which is created is due to slight intrinsic differences between the hollow cathode devices. Such differences may result in the devices contributing unequally to the total output thrust of the motor. Thus, one of the devices may be operating at a higher emission current level than the others, which current level may exceed the maximum rated level of the device. Such an unbalanced mode of operation may result in one of the devices experiencing premature failure. In an essentially inaccessible vehicle such as a spacecraft, such a failure may have a significant and permanent detrimental effect on the overall operation of the spacecraft.
In addition to premature failure, the problem of unbalanced operation may have other serious consequences in an ion propulsion motor utilizing multiple hollow cathode devices. One such consequence is an asymmetry of thrust produced by the motor. In a single cathode motor, the cathode is placed on an axis coincident with the thrust discharge of the motor. In a multiple cathode motor, however, the individual cathodes are arrayed in a symmetrical pattern about the axis of thrust discharge. Thus it may be seen that if the emission currents from each of the cathodes are not balanced, the resultant thrust will not be balanced about the thrust axis, with potentially serious adverse effects on spacecraft maneuverability.
As may be appreciated, an important component of an ion propulsion motor utilizing hollow cathode devices is a cathode emission current control system. Typically, such a control system regulates the flow of propellant through each hollow cathode relative to the voltage potential between the cathode and the anode, this potential commonly being known as the discharge voltage. The amount of propellant passing through the cathode is normally inversely proportional to the magnitude of the discharge voltage. The actual method of regulation of the flow of propellant may vary depending on the type of propellant being used. For example, in a hollow cathode device utilizing mercury as a propellant, the mercury is typically vaporized by passing it through a heated, porous plug. By varying the plug heater current, and hence the temperature, a varying amount of mercury will be vaporized, thus increasing or decreasing the flow of propellant through the hollow cathode device.
While this emission current control system may be suitable for an ion propulsion motor utilizing a single cathode, it is unsuitable for a motor utilizing two or more cathode devices, in that the individual characteristics of each cathode are not compensated for. In accordance with the foregoing example, the heater plug current of each cathode is maintained at the same level for a given discharge voltage, thus causing a substantially equal amount of propellant to flow through each cathode. The result may be that one or more cathodes in a multi-cathode motor may produce a higher emission current than the other cathodes, depending on the individual characteristics of each cathode. This unequal emission current output results in the creation of the aforementioned problems of premature failure and unbalanced thrust about the thrust axis of the motor.